After new automobiles, trucks and motor vehicles, in general, are assembled, their fuel tanks are generally filled to some extent with an appropriate fuel before the vehicles are shipped to their point of sale and delivery to the ultimate customer. Because of the global nature of the motor vehicle industry, with the assembly of the vehicles often times taking place in a different part of the world relative to the point of sale of the vehicle, the fuel that is placed in these fuel tanks often stands unused for extended periods of time during shipment and storage of the vehicles. During these periods of time, the fuel in the fuel tanks, now effectively being in storage, must retain its initial integrity and not degrade with the degradation exhibiting itself through subsequent starting and running problems in the new vehicle and also by the formation of undesirable deposits in the fuel systems of the vehicles leading to longer term operability problems. The fuel so used must resist gum and sediment formation, minimize oxidation and prevent corrosion in the metallic portions of the fuel system as well as passivate fresh metal surfaces. Likewise, the fuel storage facilities, for example, tankage, pumps and plumbing, at the motor vehicle assembly site are also susceptible to the deposition of these unwanted solid materials from the quantities of stored motor fuels awaiting transfer to the newly-assembled vehicles.
The desired stability of the fuel is usually attained through the addition of appropriate additives to the fresh fuel. Typically, complex combinations of antioxidants, such as aromatic diamines or hindered phenols, carboxylic acid-based corrosion inhibitors, and metallic ion sequesterants such as salicyclidene diamines are added as a stability-inducing package to the fuel. The term "package" is used typically to indicate the complex combination of the various stability-inducing materials often times diluted with a solvent or solvents compatible with the various individual additive materials and the fuel to be treated. This package is generally prepared as a separate entity prior to its addition to the fuel.
If such a stability-inducing additive package is not employed, spontaneous deposition of undesirable deposits of solid, insoluble materials often occurs in the fuel tanks and systems of the new vehicles. These deposits, also referred to as gum, are mainly formed from oxidized and/or polymerized hydrocarbons. If a stability-inducing additive package is employed, gum formation in new vehicles can be reduced or eliminated. However, stability-inducing additive packages that employ carboxylic acid-based corrosion inhibitors and salicylidene diamine sequesterants can lead to the formation of solid, insoluble deposits in the fuel storage and transfer apparatus of the vehicle manufacturing plant. These deposits are mainly hydrophilic carboxylate salts of metal ions arising from corrosion in the fuel storage and transfer systems and also of carboxylate salts of diamine impurities typically found in the salicyclidene diamine sequesterants. These carboxylate salts are virtually insoluble in the hydrocarbon fuel media and, as such, can also be deposited at various points throughout the fuel systems of the newly-manufactured vehicles in question.
In attempts to stabilize motor fuel toward oxidative degradation, it is often times found that sufficient oxidative stability is not imparted to the fuel by the use of a single antioxidant material. Hence, more than one specific oxidation-stabilizing material can be added to the fresh fuel to provide for the desired level of oxidative stability.